Phenomenological theory of the 1/4 commensurate phase of holmium

A model derived from a nonlocal Landau-type free energy is proposed to explain the recently observed lock-in at Q=0.25c* of the helical spin density wave vector in Ho around 96 K. The analysis suggests that Umklapp terms of the form Δ4Q,G are responsible for stabilizing the commensurate phase over a narrow temperature range. An essential requirement of the model is the existence of a mechanism to distort the polarization of the spin density from helical to elliptical. This can be achieved by a component in the basal plane of uniform magnetization or of uniaxial stress. With suitable adjustment of parameters, the measured temperature dependence of Q is reproduced.